U.S. Pat. No. 5,268,708 discloses an image processing apparatus having half-tone color proofing capabilities. This image processing apparatus is arranged to form an intended image on a sheet of thermal print media by transferring dye from a sheet of dye donor material to the thermal print media by applying a sufficient amount of thermal energy to the dye donor material to form an intended image. After the dye donor material is secured to the periphery of a vacuum imaging drum, a scanning subsystem or write engine provides the scanning function. This is accomplished by retaining the thermal print media and the dye donor material on the imaging drum while the drum is rotated past a print head that will expose the thermal print media. A translation drive then traverses the print head axially along the imaging drum, in coordinated motion with the rotating imaging drum. These movements combine to produce the intended image on the thermal print media. A scanning subsystem or write engine provides the scanning function by generating a once per revolution timing signal to data path electronics as a clock signal while the translation drive traverses the print head axially along the imaging drum in a coordinated motion with the imaging drum rotating past the print head. This is done with positional accuracy maintained, to allow precise control of the placement of each pixel, in order to produce the intended image on the thermal print media.
Although the presently known and utilized image processing apparatus is satisfactory, it is not without drawbacks. Image resolution (that is, dots imaged per inch) cannot readily be changed with existing designs. The ability to vary image resolution slightly is an advantage in that it can help to alleviate banding, moire, and other imaging effects that may occur in an image when that image is reproduced at a specific resolution. Slight changes to resolution can eliminate these effects without objectionable changes to overall image reproduction.
With existing systems, the slow scan speed, at which the print head moves along the writing drum, is fixed. As a result, the range of imaging resolutions that can be achieved using existing methods is limited, at best, to a small set of fixed values. These values are themselves dependent on maintaining stringent manufacturing and performance tolerances in components used to assemble the system. This effectively increases the complexity of manufacture, increases cost, and limits how well imaging systems can reproduce their intended targets.
Existing systems achieve their target resolutions using a precise coordination of dimensional and timing factors at the writing interface. Dimensional factors include pixel-to-pixel distance (chiefly a function of print head optics and laser diode arrangement), number of lasers used (which, in turn, determines the swath width), pitch of the lead screw, and writing drum circumference. Timing factors include drum rotation speed and the linear motion of the print head as it moves along the writing drum. Each of these above named factors are tightly coupled and are highly inter-dependent. Taken together, these factors determine the addressability of individual points on the imaging surface, thereby determining what resolution values are achievable for the image processing system.
The tight coupling of the dimensional and timing factors listed above makes it difficult to adapt an image processing system of this type for different resolutions and for different swath widths. To change from one resolution to another, or from one swath width to another, requires corresponding changes in more than one of the factors listed above. Certain of these factors are fixed and cannot be changed once the image processing apparatus is built. For example, the writing drum circumference and the pitch of the lead screw are fixed. Changing any of the other factors requires corresponding changes to effect the intended output resolution. For example, changing the laser spacing or number of lasers that write simultaneously changes the swath width and requires corresponding changes to the linear motion of the print head along the vacuum imaging drum.
The print head linear motion is itself a factor of the rotational speed of the drive motor for the lead screw and lead screw pitch. However, the capability to vary this speed for precision operation over a range of values requires an expensive motor and it can be difficult to maintain consistent results using this method.
As a result of these tightly coupled factors, the ability to alter the imaging resolution and swath width for multiple values requires the ability to adjust both dimensional and timing characteristics of the image processing apparatus.
The above description applies for an image processing apparatus that uses a single station for imaging. However, this same design can be extended to cover an image processing device that utilizes more than one imaging station, such as a printing press, where each station images using a different color. With such a design, it is important that each station be properly adjusted to provide accurate registration in reproducing successive color separations. Minor tolerance differences in head positioning can lead to objectionable effects and registration errors in the output image, since swath width errors are additive along the path of the print head.
There are additional concerns for variation in head traversal between imaging machines. Separate image processing machines at the same site need to be adjusted so that they provide similar registration and results for the same image. Manufacturing tolerances prevent two machines from providing precise positioning relative to each other without some method for fine-tuning the head traversal mechanism.
The overall method of using a lead screw mechanism for tight control of writing component positioning is well-known in the art. In particular, systems that use magnetic read/write heads use lead screw mechanisms (for example, see U.S. Pat. No. 4,270,155, U.S. Pat. No. 4,313,143, and U.S. Pat. No. 4,747,004). Existing patents also show the use of the guide rod mechanism in conjunction with the lead screw for maintaining proper head alignment and positioning that compensates for lead screw tolerance differences (see U.S. Pat. No. RE:33,661).